This invention relates to electrolysis of molten alkali hydroxide and particularly to depletion of water formed therein.
An ideally energy efficient cell for electrolyzing molten sodium hydroxide would operate at the reversible voltage to yield the decomposition EQU 2NaOH.fwdarw.2Na+H.sub.2 O+1/2O.sub.2
with sodium forming on the cathode to dissolve in adjacent catholyte, with water forming on the anode to dissolve in adjacent anolyte, and with oxygen also forming on the anode in a gaseous phase which separates from the anolyte. This ideal cell would have an energy efficiency of 100% but Castner cells, which were an early commercial source of sodium metal, had an energy efficiency of less than 25%. These cells, disclosed by H. Y. Castner in U.S. Pat. No. 452,030 (1891), used a nickel gauze diaphragm to separate anolyte from catholyte to prevent their mixing which would have reduced decomposition yield to negligible quantities. But despite the diaphragm and operation near the electrolyte melting temperature which reduced solubility of the sodium, the accumulation of sodium and water to saturation and their diffusion toward opposite electrodes resulted in the secondary reaction EQU Na+H.sub.2 O.fwdarw.NaOH+1/2H.sub.2
so that the two moles of sodium produced according to the decomposition of the ideal cell were diminished to one mole thereby limiting current efficiency of the Castner cells to 50%. Since ohmic loss in electrolyte between electrodes results in a voltage efficiency of about 50%, the expected energy efficiency of Castner cells is about 25%. Energy efficiency can thus be improved by reducing the parasitic reaction of sodium with water and by reducing interelectrode distance.
The secondary reaction may be reduced by depleting either the anolyte or the catholyte or both of their decomposition product which, because of the adverse effect on efficiency, is regarded as an impurity. One method for reducing anolyte water content is described by F. J. Dobrovolny in "Official Gazette" 1950, Vol. 637, pages 1575-6. The anolyte is circulated through a heating zone where it is flushed with an inert gas to remove water vapor. Current efficiency is substantially improved but thermal dehydration is not an energy efficient means for attaining low concentrations of water in sodium hydroxide. Another method for reducing anolyte water content, disclosed in the cited parent application, comprises circulating the anolyte through a second electrolysis cell which operates at a voltage sufficient to decompose water but not sufficient to decompose sodium hydroxide. Although such decomposition of water requires the same current as would be used to decompose the sodium hydroxide formed by the sodium-water parasitic reaction, the second cell operates at a lower voltage for improved energy efficiency. Overall energy efficiency of the dehydration process is further improved by transporting the hydrogen and oxygen decomposition products to a fuel cell for partial recovery of the electrical energy used to electrolyze the water.
An electrolysis cell with reduced interelectrode distance was disclosed by R. G. Cottam et al. in U.S. Pat. No. 3,242,059 wherein porous electrodes are adjacent to a diaphragm so that the interelectrode distance is the thickness of the diaphragm. The cell is a chlor-alkali diaphragm cell through which an aqueous solution of sodium chloride percolates. Chlorine gas forming on the porous anode passes therethrough and is thereby removed from operating portions of the cell. Sodium forming on the cathode reacts with water to form sodium hydroxide and hydrogen, both of which pass through the porous cathode. The porous electrodes enable reduced interelectrode distance with a consequent improvement of voltage efficiency, but the structure of Cottam's cell is not appropriate for drawing electrolyte through the porous electrode so that impurity therein can be depleted and the electrolyte returned into the cell for further electrolysis with a reduced parasitic reaction and improved current efficiency.